1. Field of the Invention
This invention relates to a correlator and control circuit for a vehicle to process signals useful in a vehicular collision avoidance system, more particularly, of the SECANT type for aircraft.
2. Description of the Prior Art
Collision avoidance systems have been proposed as means of preventing collisions by vehicles both of the maritime vessel type and of the airborne type. Cooperative systems of the asynchronous type include interrogator-transponder techniques in which interrogation signals or probes are transmitted from one vehicle on a random basis. Vehicles in the vicinity receiving such probes respond with signals of various indicia to provide to the interrogation vessel information relating to both range and information identifying the vessel and its position. In airborne systems such information may include the relative or actual altitude of the aircraft.
An existing collision avoidance system known by the acronym SECANT (Separation Control of Aircraft by NonSynchronous Techniques) employs probes identified by any one of a plurality of frequencies and replies using different frequencies of the same band but arranged into a predetermined correspondence to a particular probe frequency. Special correlation techniques separate the true reply from other signals which may be probes from remote vessels generally identified as "fruit." Such a system is described in U.S. Pat. No. 3,755,811 issued Aug. 28, 1973, and 3,803,608 issued Apr. 9, 1974, based on the inventions of Jack Breckman.
A variation of the SECANT system known by the acronym VECAS (Vertical Escape Collision Avoidance System) employs the basic principles of the SECANT feature incorporating the discriminant for identifying and tracking targets based on relative altitudes of the interrogating (or transponding) and replying aircraft. For such a system, altitude information is provided with a sequential series of the reply signals in accordance with the system described in the above-identified patent application, Ser. No. 462,491, filed Apr. 19, 1974, entitled "Altitude Coding for Collision Avoidance System," based on the invention of J. J. Lyon now the aformentioned U.S. Pat. No. 3,947,845.